Want to send all outgoing mail through a single SMTP relay (e.g. SMTP2GO) while using different IMAP intake addresses (e.g. s1@relay.com for intake but support@domain.com as sender).

osTicket requires each email address to be added as a system email in order to send from it. The recommended approach is to add the public sender addresses (support@domain.com, etc.) as system emails in osTicket and configure SMTP on each one pointing to your relay. For intake, configure a separate IMAP mailbox on each email. Alternatively, configure PHP's local sendmail/postfix to route outbound mail through the relay and rewrite the sender address at the MTA level, then skip per-email SMTP config in osTicket.